The present invention relates to a paper machine multiple cylinder dryer.
The paper machine multiple cylinder dryer includes a plurality of heatable drying cylinders and corresponding rollers. A drying fabric supports the web, passing from one cylinder or roller to another in closed conduction. The cyclic path of the drying fabric is such that part of the drying cylinders or rollers remain within the loop of the fabric and part remain outside the fabric. A supporting fabric carries the web on the cylinders and rollers. The supporting fabric is guided by guide rollers. Elongated pockets present in the dryer are defined, in conjunction with the cylinders, by the web runs supported by the drying fabric, the free surfaces of the drying cylinders and the run of the supporting fabric.
Web breaks in the drying section of fast-running paper machines constitute an essential factor limiting the operation of such machines at higher speeds.
The term "closed conduction" is understood herein to mean a so-called single felt conduction, wherein one or a plurality of rollers or cylinders of a drying cylinder group are located within a felt loop implementing the single felt conduction and the other cylinder is, or the other cylinders are, outside the loop. Although fabrics are called "felts" herein, the word "felts" is understood to mean any type of usable fabrics such as, for example, reticular plastic fabrics.
Single felt conduction is in itself a technique known in the art. For example, U.S. Pat. No. 796,601 discloses a felt run pattern for single felt conduction. However, to the inventors' knowledge, single felt conduction was not introduced in commercial use until about 70 years after it was invented. The applications of single felt conduction have primarily been fast-running machines. In such cases, however, pressure is generated in the closing-down nips on those drying cylinders where the web lies outermost. This pressure tends to detach the web from the fabric and, in fact, partly does so. The air trapped between the fabric and the web on those drying cylinders where the web lies between the cylinder and the felt is thereby compelled to escape, in the closing-down nip, from between the web and the fabric. This tends to produce a so-called blowing effect on the web. Of the aforementioned phenomena, the former requires a felt as impermeable as possible and the latter requires one with highest possible permeability. For this reason, despite endeavors to provide optimum felt permeability, the aforedescribed phenomena promote the occurrence of breaks and impose a limit on the speed increase. Furthermore, optimum felt permeability does not yield the best possible evaporation. In addition, the web must carry and maintain, when outermost on the cylinder arc, stresses arising from centrifugal forces. When the velocity increases past 1000 m/min, these stresses come close to the original wet strength of the web in fast-running paper machines, with typical pulps.
Finnish patent application No. 780333 of J. M. Voith GmbH discloses the installation of a flow obstacle in the nip in question, in an effort to eliminate the blowing effect in a nip where the web lies outermost. However, this procedure of the known art fails to eliminate the blowing effect exerted on the web by a closing-down nip where the felt is outermost. Furthermore, this procedure does not possibly provide support for the web when the web is outermost on the cylinder arc.
Conduction of the web between two fabrics throughout the time of cylinder drying or blow-on drying, is also known in the prior art, as disclosed in U.S. Pat. Nos. 1,122,282; 3,378,936 and 3,576,078.
Both of the aforementioned known arrangements cause a substantial reduction in the drying effect. This is a serious drawback, since drying is, of course, the primary task and object of the drying section.
It has also been suggested that differential pressure be utilized to support the web in connection with single felt conduction, while it lies outermost on a cylinder. Thus, for example, Finnish patent application No. 771056 (corresponding to U.S. Pat. No. 4,202,113) and No. 793643 disclose the application of differential pressure to those cylinders where the web is outermost, through the recessed surfaces of the respective cylinders. If required, sub-atmospheric pressure is then also applied to the periphery of the cylinder. The supporting effect of the application of sub-atmospheric pressure also extends to the smooth cylinder and to the free draw between cylinders. However, the solutions hereinbefore described imply special vacuum arrangements, and their drawbacks include sealing problems and extra power consumption.
U.S. Pat. No. 4,000,035 discloses drying and forwarding the web in conjunction therewith in outermost position on the cylinder with the aid of through-flow drying. This type of drying produces pressure urging the web toward the felt. This particular arrangement also features a pull-in strip movable in the cross-web direction on the cylinders by special dispositions. A drawback of the arrangement of U.S. Pat. No. 4,000,035 is that the end product produced by the paper machine, excluding so-called tissue brands, has a permeability so low that the effect of through-flow drying is poor. Furthermore, evaporation achieved in this manner is exceedingly costly.
Finnish patent application No. 761953 (corresponding to U.S. Pat. No. 4,183,148) and Finnish patent No. 54,954 (corresponding to U.S. Pat. No. 4,172,007) disclose the use of a supporting fabric in connection with single felt conduction in order to try to prevent web detachment from those cylinders on which the web lies outermost. Furthermore, said Finnish patent discloses a blowing effect to urge the web against the cylinder, in those sectors where the web lies outermost and where the fabric does not support the web. However, the procedure disclosed by said Finnish patent cannot prevent the web from separating from the felt on the free draws. Furthermore, it is impossible to prevent the web from following along with the cylinder in those opening-up clefts where the cylinder abuts the web.
It is also previously known from drying groups with normal web conduction that the pressure and flow conditions in the pocket may be influenced, in the first place, by the running speed, felt permeability and location of the felt guide roller, as disclosed in Finnish patents No. 44332 and 45884 of the present inventors. However, the object of said Finnish patents is to maintain the pressure in the pocket as high as the atmospheric pressure prevailing on the ends of the cylinders. Furthermore, the inventions of said Finnish patents are intended for use in connection with standard conduction.